1. Field of the Invention
The invention concerns a process for producing an erasable printing form, a method for erasing the printing form, the printing form itself, as well as its use for coating a roller.
2. Description of the Prior Art
In modern offset printing processes which employ a moistening agent to moisten the printing form, a photopolymer is applied to a hydrophilic (water-accepting) surface of the printing form and is first exposed and then developed to form images. In so doing, hydrophobic (ink-accepting) image locations corresponding to an image to be printed are left behind on the surface of the printing form, while the photopolymer is removed from the non-image locations. The surface of the material forming the printing form is freed again at the non-image locations as a result of the developing step. The printing form is formed e.g. by a roughened aluminum plate on which an aluminum oxide (Al.sub.2 O.sub.3) layer has been applied anodically. The aluminum oxide layer has a porous surface which promotes the adhesion of a hydrophobic coating and, moreover, stores moistening agent in the pores and accordingly improves the hydrophilic properties.
However, this aluminum oxide layer has the disadvantage that a polymer layer which is applied to it can be removed again only with difficulty if the printing form is to be used repeatedly. In order to clean the capillaries in the surface of the aluminum oxide layer so as to rid them of residues interfering with a subsequent printing process, deep-acting cleansers must be used in a lengthy cleaning process. Under certain circumstances, these cleansers could also attack the aluminum oxide layer and shorten its useful life.
On the other hand, a printing form is known from DE 36 33 758 A1 on which hydrophobic and hydrophilic areas can be formed and which contains a material with ferroelectric characteristics. This material can be polarized and depolarized in selected areas or can be brought to the three different polarization states (positive or negative polarization or depolarization). The printing form is polarized by applying an electrical d.c. voltage to an electrode and using an electrically conductive layer beneath the ferroelectric material as a counter-electrode.
Conversely, the printing form can be depolarized again by means of alternating voltage whose frequency is far greater than the natural or resonant frequency of the ferroelectric material or by heating to a temperature above the Curie temperature or can be uniformly polarized again by subsequent application of a d.c. voltage. Barium titanate, or example, whose Curie temperature is greater than 120.degree. C., can be used as a ferroelectric material. Other materials having ferroelectric properties can also be used instead of barium titanate, e.g. a composite material with hydrophobic properties such as soft-plastic matting with embedded ferroelectric micro-crystallites.
However, a disadvantage in the previous known for methods for rendering printing forms with ferroelectric properties reusable for offset printing is that either a current source and a counter-electrode must be brought to the printing form or a heat source is required. The ferroelectric layers must be thin to prevent unnecessarily high electrical voltage.